The model misses comets, and accretion in general.

For example, asteroids may be limited to a very small number of solar systems. The effects of comets will also vary according to the initial accretion of any solar system.

Both of these processes are very very significant if one is trying to find planets where intelligence evolved. Of course, the best way to obtain the data necessary for such a search is with Moon based facilities, but that's another topic...

As Monty Python pointed out, we better hope that there's intelligent life somewhere out there, because there's damn sure little of it here. If there were, we'd be dealing properly with the impact
hazard.

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